1. Field of the Invention
The present invention relates to a surface level detection method, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
The recent shortening of the wavelength of light used for an exposure apparatus (to be referred to as exposure light, hereafter) has improved the resolution of an exposure apparatus. Along with such a trend, a demand has arisen for enhancing the accuracy of detecting the surface level of a wafer to be exposed (the accuracy of executing a focus measurement).
Normally, a sensor for detecting the wafer surface level is positioned on the forward side of an exposure slit. Using the sensor, the wafer surface level is detected (measurement process is executed) immediately before an exposure process. A wafer stage is controlled at an exposure position such that the wafer surface level coincides with the focal plane of a projection lens.
To accurately detect (measure) the wafer surface level, it is desirable to measure a large number of measurement points on the wafer surface at a very narrow pitch. In such a circumstance, the throughput of an exposure step, including a measurement process and an exposure process, may decrease due to an increase in measurement time.
To prevent this problem, a twin-stage type exposure apparatus is proposed, which comprises a first wafer stage on which a wafer is exposed and a second wafer stage on which a wafer is measured. The first wafer stage is arranged in an exposure station, while the second wafer stage is arranged in a measurement station. In the measurement station, a focus detection system for detecting the wafer surface level, and an alignment detection system for measuring the alignment position of a wafer exposure region are set. The twin-stage type exposure apparatus can measure a wafer in the measurement station, while exposing another wafer in the exposure station in parallel. Doing so makes it possible to simultaneously improve the accuracy of detecting the wafer surface level and the exposure step throughput (for a plurality of wafers).
The focus detection system of the twin-stage type exposure apparatus generally adopts a focal plane detection unit for detecting a focal plane of exposure light. The focal plane detection unit causes a light-projecting system to obliquely irradiate a target detection surface (wafer surface) with a slit-like light beam from above. Then, a slit-like optical image (to be referred to as a slit image hereafter) is formed on the target detection surface. The light beam reflected by the target detection surface is guided to re-form a slit image on a light-receiving system (e.g., a CCD area sensor). The re-formed slit image will be called a target detection image hereafter. The light-receiving system monitors the position of the target detection image thereon. With this operation, the position of the target detection image formed on the light-receiving system changes as the target detection surface (wafer surface) is displaced vertically. The focal plane detection unit detects the wafer surface level by monitoring the position of the target detection image formed on the light-receiving system.
There is a case when an error caused upon detecting the wafer surface level by the light-receiving system may increase. Assume, for example, that a thin film, such as a photoresist, adheres to the wafer surface. A light beam, obtained by interference between light reflected by the surface of the thin film and light transmitted through it and reflected by the wafer surface, may form an image on the light-receiving system. Such a circumstance increases an error caused upon detecting the wafer surface level by the light-receiving system.
Under such a circumstance, Japanese Patent Laid-Open No. 2002-334826 proposed the following technique. That is, the light-receiving system moves to a position along the wafer surface, which is determined using layout information, to detect the surface level at the position. An offset is then calculated from the detected value to correct, based on the calculated offset, the detection result (focus measurement value) obtained by the light-receiving system.
However, the technique disclosed in Japanese Patent Laid-Open No. 2002-334826 does not consider the fact that the relative positions, each of which is of a focus measurement region relative to a target measurement region in a shot region, are displaced among shot regions. This fact makes it impossible to correctly calculate an offset for correcting a focus measurement value. The target measurement region means a region on the focal plane, where a slit image is desirably formed, and which has a constant relative position to each shot region along the surface. The focus measurement region means a region where a slit image is actually formed. Therefore, it may be impossible to accurately detect the wafer surface level.
Assume, for example, that a slit image is formed on a focus measurement region MA1 spaced apart from a target measurement region OA in a shot region SAa, as shown in FIG. 24. The focus measurement region MA1 exists on a first horizontal plane HP1 and on the surface of a thin layer 5a. That is, when the focal plane detection unit irradiates the focus measurement region MA1 with a light beam LF1, the light-receiving system receives a light beam LF2 reflected by the surface of the thin layer 5a and a light beam LF3 reflected by the interface of a high-reflectance layer 5c. The intensity of light applied to the light-receiving system exhibits a distribution as indicated by a waveform DW1. Specifying the position of a target detection image at a peak position DP1 of the waveform DW1 makes it possible to detect the vertical position of a wafer WF.
Assume, for example, that a slit image is formed on a focus measurement region MA2 in a shot region SAb, as shown in FIG. 25. The positions of the focus measurement regions MA1 and MA2 relative to the target measurement region OA are displaced along the surface by ER1. Both the focus measurement regions MA1 and MA2 exist on the first horizontal plane HP1. The relative positions of the focus measurement regions MA1 and MA2 relative to the target measurement region OA are not displaced in the vertical direction. That is, when the focal plane detection unit irradiates the focus measurement region MA2 with the light beam LF1, the light-receiving system receives a light beam LF4 reflected by the surface of the thin layer 5a and a light beam LF5 reflected by the interface of a low-reflectance layer 5b. The intensity of light imaged on the light-receiving system exhibits a distribution, as indicated by a waveform DW2. The intensity of the light beam LF5 is lower than the intensity of the light beam LF3 (see FIG. 24). Specifying the position of a target detection image at a peak position DP2 of the waveform DW2 causes a displacement ER2 from the peak position DP1. It is, therefore, impossible to correctly calculate an offset between the shot regions SAa and SAb. Such a circumstance leads to a failure in accurate detection of the vertical position of the wafer WF.
Assume that a slit image is formed on a focus measurement region MA3 in a shot region SAc, as shown in FIG. 26. The relative positions of the focus measurement regions MA1 and MA3 relative to the target measurement region OA are displaced along the surface by ER1 and in the vertical direction by ER3. The focus measurement region MA3 exists on a second horizontal plane HP2 spaced apart from the first horizontal plane HP1 by the distance ER3. That is, when the focal plane detection unit irradiates the focus measurement region MA3 with the light beam LF1, the light-receiving system receives a light beam LF6 reflected by the surface of the thin layer 5a and a light beam LF7 reflected by the interface of the low-reflectance layer 5b. The intensity of light imaged on the light-receiving system exhibits a distribution as indicated by a waveform DW3. The intensity of the light beam LF7 is lower than the intensity of the light beam LF3 (see FIG. 24). Specifying the position of a target detection image at a peak position DP3 of the waveform DW3 causes a displacement ER4 from the peak position DP1. It is, therefore, impossible to correctly calculate an offset between the shot regions SAa and SAc. Such a circumstance leads to a failure in accurate detection of the vertical position of the wafer WF.